Use of imaging sensors or ultrasonic sensors or radar sensors in vehicle sensing systems is common and known. Examples of such known systems are described in U.S. Pat. Nos. 8,013,780 and 5,949,331 and/or U.S. publication No. US-2010-0245066 and/or International Publication No. WO 2011/090484, which are hereby incorporated herein by reference in their entireties.
Ultrasonic sensors have been widely deployed for use in ultrasonic parking systems since the late 1990s. Systems have progressed from provided functions such as rear parking aid, to include automated parking assist, blind spot detection, and the like, adding increasing numbers of sensors per vehicle. Since their introduction, the design and function of ultrasonic systems have become standardized, with periodic incremental improvements occurring to support longer range, improved accuracy and positioning of objects within the desired system field of coverage, and in near range detection. The technology involves transmission cycles where an ultrasonic burst at a defined frequency (35-60 kHz has been typically used) is emitted, followed by a receive cycle. The duration of this transmit-receive cycle is a function of the speed of sound. As a result, longer range detections inherently create longer system update times, where current system update rates are less than five Hz (see FIG. 1), nearly half of the speed of early short range systems.
To support automated functions, higher accuracy of measurement is required. Ultrasonic transmission range calculations are based on the speed of sound through air. This is highly influenced by temperature and humidity, with impacts of approximately five percent for twenty degrees C. and fifty percent relative humidity change from room temperature, fifty percent humidity conditions (see FIG. 2).
The ultrasonic transducer vibrates to generate the burst when transmitting. Before the transducer can receive signals, the transducer's vibration must be damped, so that low energy received signals can be detected. This typically requires 1 ms or greater (see 1 in FIG. 3), causing the system to be unable to detect objects closer than about 17 cm (c=343.37 m/s at twenty degrees C.) from the face of the sensor.
An ultrasonic sensor's typical field of view (FOV) is 120 degrees horizontal and 60 degrees vertical (see 2 and 3, respectively, in FIG. 4). Where used for parking slot measurement, a narrower field of view is desired to improve the accuracy in measurement of potential open parking space, resulting in about a 60 degree horizontal and 60 degree vertical FOV (see 4 and 5, respectively, in FIG. 5).
Objects detected by the ultrasonic system provide a single range measurement, with no angular position available from a single sensor. To more accurately position or determine the location of objects relative to the vehicle, multiple sensors are positioned along the vehicle's bumper fascia so that triangulation techniques can be used by a central Electrical Control Unit (ECU) to calculate the position of objects (see FIG. 6). By using reflections of transmissions sent by each sensor and received by multiple sensors 6 (1st_E11, 1st_E10, 1st_E01, 1st_E00), the accuracy of detection can be improved to ±1 cm. Ultrasonic sensors currently used in practice use multiple sensors, which results in multiple visible transducer faces (approximately 15.5 mm diameter).